Modeling and mitigation of the magnetic field errors in RFX-mod Upgrade

RFX [1] was originally designed with a load assembly consisting of a vacuum vessel (VV) and a thick aluminum stabilizing shell, with two poloidal and two equatorial cuts (i.e. gaps). After several years of experimental campaigns, a major modification of the RFX load assembly has been introduced [2], consisting in the substitution of the aluminum shell with a thin Copper Shell (CS) and the installation of a stainless steel Toroidal Supporting Structure (TSS). At the same time, the machine has been equipped with an innovative active control system of MHD modes [3]. After a relatively long period of successful operations, further modifications of the magnetic front-end of RFX-mod have been recently considered. In particular, the VV will be removed with the aim of improving passive MHD control (plasma-shell proximity increase) and plasma rotation (preventing the braking torque caused by the VV itself). In this paper, a detailed analysis of the new magnetic front-end is presented, with particular emphasis on modeling of the magnetic field errors generated at the poloidal gaps (butt-joint configuration) during the transient phases of the discharge. A non linear equilibrium code, MAXFEA [4], has been used to simulate a reference scenario and provide the time evolution of PF coils and plasma currents as input of the 3D FEM analyses. A set of gap correction coils, previously installed in RFX [5], have also been included in the model to assess its capacity to minimize the error field at plasma edge, complementing the work presented in [6]. [1] Topical issue on RFX-experiment, FED 25 (1995)[2] P. Sonato, FED 66-68 (2003) 161-168 [3] G. Marchiori, FED 82 (2007) 1015-1022[4] P. Bettini, NF 43 (2003) 119-129[5] F. Bellina, FT 1 (1997) 771-775[6] P. Bettini, FED 96-97 (2015) 649-653